This invention relates to infrared gas analyzers and, more particularly, to an improved infrared gas analyzer capable of high accuracy and fast response, yet still relatively low in cost.
Many types of infrared gas analyzers utilize an infrared source to produce and direct infrared energy through an unknown gas mixture contained in a sample cell. The energy passing through the sample cell is detected and electrical signals are produced representative thereof. These signals are processed to produce an output indicating the concentration of one or more of the constituents of the gas mixture in the sample cell.
Such gas analyzers utilize the principle that various gases exhibit a substantial absorption characteristic at specific wavelengths in the infrared radiation spectrum. A gas analyzer of this type is shown and described in U.S. Pat. No. 4,013,260, McClatchie, et al. issued Mar. 22, 1977, and assigned to the assignee of the present invention. Another type of infrared gas analyzer is shown and described in U.S. Pat. No. 3,953,734, Dimeff, issued Apr. 27, 1976, and assigned to the United States of America.
In both of the above cited patents, and in similar types of infrared gas analyzers, the wavelength band of the beam of infrared energy passing through the sample cell containing the unknown gas mixture is changed periodically by the interposition of one or more filters in the path of the light beam. Typically, each filter passes only radiation at a characteristic absorption wavelength band of a particular gas of interest. Another filter may also be used as a reference filter at a wavelength band close to but not substantially overlapping the characteristic absorption wavelength band of any of the gases present in the sample cell.
Gas analyzers of the foregoing described type usually continuously reference the radiation detected at the characteristic bands to radiation detected at reference levels (i.e., a non-absorbed wavelength and a dark or totally blocked level). By doing so, the effect of so-called drift is minimized, and the effect of background noise is reduced. Drift can occur as a result of contamination on the windows in the sample cell which will attenuate the radiation passing therethrough and which could be interpreted erroneously as indicating the presence of the gas to be detected in the gas sample. Drift can also be caused by shifts in the output of the detector, inherent in many detector constructions, and temperature changes in the source of the infrared radiation. The process of correcting for drift is sometimes referred to as span stabilization.
One such analyzer is described in Passaro, et al., U.S. Pat. No. 4,346,296. In this disclosure, an infrared source emits infrared radiation at relatively constant intensity over a relatively broad spectrum. The infrared radiation from the source is interrupted periodically by a chopper wheel. After passing through the sample cell, the chopped infrared radiation is detected by respective detectors. In each case the radiation is filtered by a narrow passband filter so that each detector is effectively sensitive only to the radiation of a particular narrow band of frequencies corresponding to a respective absorption frequency characteristic of one of the respective gases. The respective detection signals are thus systematically related to the concentration of each of the respective gases. Because of the chopper wheel, these signals are AC signals at the chopper wheel frequency. The signals are then amplified, detected and filtered to produce corresponding DC signals.
Gas analyzers that employ multiple filters, rotating filter wheels, and/or chopper wheels or the like require a number of relatively expensive components, are subject to failure as a consequence of rapidly rotating parts, and are relatively large in size. It is desirable to provide a gas analyzer design in which size, failure likelihood, and rapidly rotating parts are reduced or eliminated.
Accordingly, it is an object of the present invention to provide an improved infrared spectrophotometer for analyzing gases.
Another object of the invention is to provide an improved gas analyzer which is relatively low in cost.
Another object of the invention is to provide an improved gas analyzer which is relatively smaller in size.
These and other objects of the invention will be understood more particularly from the following description taken in connection with the accompanying drawings wherein: